U.S. Pat. No. 4,446,602 discloses a light beam fence with a single beam reflected by mirrors to detect the presence of a person within a zone and to stop the operation of the machine in the zone. U.S. Pat. Nos. 3,730,633 and 4,021,119 disclose an optical system for measuring the amount of intrusion of an object into a light beam split into two beams directed by mirrors. U.S. Pat. No. 4,811,004 discloses an optical position sensing system for computer displays. The system uses light beams scanned by moving mirrors. U.S. Pat. No. 4,564,085 discloses a light beam detecting system with a mirror which may swing out of the way to allow an alternate light path to be used.